Firefox 54 (scheduled on 2017-06-13) will require cargo 0.16.0 and in future you may want to chase lang/rust updates. http://searchfox.org/mozilla-central/rev/c9ccb8ec1f5a/build/moz.configure/rust.configure#59
Hi! Sorry for the delay, I'm preparing the patch this week-end.
I uploaded a patch to Phabricator: https://reviews.freebsd.org/D9906
A commit references this bug: Author: dumbbell Date: Thu Mar 23 08:48:00 UTC 2017 New revision: 436758 URL: https://svnweb.freebsd.org/changeset/ports/436758 Log: devel/cargo: Update to 0.17.0 Beside the new upstream release, the port was cleaned up a bit to be closer to the lang/rust* ports. PR: 217338 Reported by: jbeich Approved by: antoine Differential Revision: https://reviews.freebsd.org/D9906 Changes: head/devel/cargo/Makefile head/devel/cargo/distinfo head/devel/cargo/distinfo.amd64 head/devel/cargo/distinfo.i386 head/devel/cargo/files/ head/devel/cargo/pkg-plist